Function futures_util::future::select

Important traits for Select<A, B>

impl<A, B> Future for Select<A, B> where
    A: Future + Unpin,
    B: Future + Unpin,  type Output = Either<(A::Output, B), (B::Output, A)>;

pub fn select<A, B>(future1: A, future2: B) -> Select<A, B> where
    A: Future + Unpin,
    B: Future + Unpin, 

Waits for either one of two differently-typed futures to complete.

This function will return a new future which awaits for either one of both futures to complete. The returned future will finish with both the value resolved and a future representing the completion of the other work.

Note that this function consumes the receiving futures and returns a wrapped version of them.

Also note that if both this and the second future have the same output type you can use the Either::factor_first method to conveniently extract out the value at the end.

Examples

use futures::future::{self, Either, Future, FutureExt};

// A poor-man's join implemented on top of select

fn join<A, B, E>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
    where A: Future + Unpin,
          B: Future + Unpin,
{
    future::select(a, b).then(|either| {
        match either {
            Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
            Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
        }
    })
}